This invention relates to magnetic shielding for a current sensor of direct current switching apparatus which utilizes permanent magnet fields for driving an arc off the contacts into an arc splitter structure. More particularly, the invention relates to magnetic shielding means for a current sensor to negate affects of the permanent magnets on the current sensing means.
Apparatus for switching high voltage D.C. power is disclosed and claimed in U.S. Pat. No. 5,004,874 issued Apr. 2, 1991, the disclosure of which is incorporated herein by reference, and which patent is assigned to the assignee of the present invention. The D.C. switching apparatus disclosed in the aforementioned patent comprises an electromagnetically operated contactor wherein a movable bridging contact is driven into and out of engagement with a pair of stationary contacts mounted on contact supports which are connected with external terminals connectable to a power supply. The stationary contacts are disposed in arc chambers which comprise a plurality of splitter plates and arc runner surfaces leading from the contact to the splitter plates. Permanent magnets are disposed externally of the arc chambers to establish a magnetic field through the respective arc chamber, which magnetic field co-acts with current in an arc drawn between movable and stationary contacts to create a force which moves the arc from the respective contacts into the arc splitter plates for interruption of the arc. The switching apparatus is disclosed as being contained in a sealed envelope which may contain a control electronics module and ancillary snap action switches or the like which are electrically connected to a multi-pin connector module mounted to the envelope.
The D.C. switching apparatus may incorporate a current sensor connected to the control electronics module for operating the D.C. switching apparatus to an open condition upon detection of a predetermined current level flowing through the switching apparatus. Such current sensors commonly convert a current induced magnetic field to an analog voltage output signal. Such output signal may be utilized by measuring apparatus to provide a visual readout of the voltage, by control apparatus to initiate some function or by a microprocessor as an input to a larger system. In switching apparatus of the aforementioned type, the current sensor can be incorporated to provide over-current protection in the apparatus wherein the output signal of the sensor is applied to the control electronics module which in turn controls the electromagnet of the switching apparatus to cause the movable contacts to be moved out of engagement with the stationary contacts, thereby opening the circuit in the function of a circuit breaker. However, the relationship and magnitude of the magnetic field induced by current flow in the contactor past the transducer is necessarily very sensitive to obtain the desired accuracy. Such accuracy can be particularly affected in D.C. switching apparatus of the aforementioned type wherein permanent magnets are utilized to move the arc from the contacts into the arc splitter plates. Such permanent magnets can also create fringing flux patterns which can adversely impact the magnetic field seen by the transducer to provide inaccurate current readings.
A current sensor for switching apparatus of the aforementioned type requires particular attention to such details as location, orientation, and magnetic shielding. When the switching apparatus is utilized to provide over-current protection such as a circuit breaker, it is also important that the analog output signal of the current sensor be linear throughout the entire range of current levels to be detected.